(1) Field of the Invention
The present invention relates generally to optical phase signal reception. In particular, a method and device for reception of a polarization and phase varying optical phase signal without output power fade is provided.
(2) Description of the Prior Art
Devices for the reception of optical signals are well known in the prior art. Included in the prior art are devices which use photodetectors to provide an electric output signal proportional to the incident optical signal power. In many of these devices, fading due to variations in phase and polarization is avoided by incoherent optical detection. However, the elimination of polarization fading by incoherent detection comes at a cost. First, the strength of the output signal varies as the square of the input signal power. This variation means that for every decibel of input signal power lost in a receiver system, two decibels of receiver output current are lost. This square law characteristic has limited incoherent optical receivers in the prior art to dynamic ranges of less than 80 dB and optical detection noise floors to greater than -80 dBm (dB referenced to 1 milliwatt) per Hertz bandwidth. Moreover, the phase characteristics of the incident optical signal cannot be determined by this type of optical receiver.
Coherent optical receivers, on the other hand, remedy many of the problems of the earlier incoherent receivers. Typically, coherent optical receivers combine the input optical signal with a signal produced by a local oscillator to form an interference. The interference between the two optical waves produces an optical "beat" which can be used to measure the phase difference between the signal and the local oscillator. This "beat" is as a result of the square law nature of the optical detectors. The earliest coherent optical receivers, known in the art as homodyne. receivers, use a local oscillator with the same frequency as the input optical signal. This method allows the detection of the phase characteristics of the incoming signal; however, the output signal is subject to fading caused by either the phase or the polarity of the input signal. Later, heterodyne receivers were developed which used a local oscillator with a frequency different from that of the incoming signal to eliminate fading due to phase. Nevertheless, fading, due to polarization, remains a problem in the art.
Prior art phase detectors have relied upon the capabilities of the underlying optical receiver. These phase demodulation oriented optical receivers have historically been limited in two ways. First, the inability of the prior art to produce a phase and polarization fading-free receptor has caused the performance of prior art phase detectors to be sensitive to variations in phase and polarization of the input optical signal.
Additionally, prior art efforts have required constant output power to input power ratios--in order to achieve this, the output voltage is normally clipped or otherwise electronically limited. The use of these clippers or limiters in the prior art causes signal to noise ratios in the detector to vary over time.
No prior art methods or devices exist which provide phase and polarization fading-free demodulation of phase information while maintaining a constant, nontemporally-varying signal to noise ratio.